(1.) Field of the Invention
The present invention relates generally to devices for lifting and precision positioning of objects. Specifically, it is a device for positioning the lifting force directly over the lifted object's center of gravity and providing a means for rotating the device through slight displacements of the lift force from the object's center of gravity.
(2.) Description of the Prior Art
During testing of equipment at the Naval Undersea Warfare Center, it is often necessary to suspend a test piece within a test tank. To do this, the test piece must be inserted horizontally into the test tank. The means for lifting these heavy test pieces is situated above the tank, and is commonly attached to the test piece from above. However, as the test piece is slid into the mouth of the test tank, the center of gravity of the test piece moves inside the test tank. Thus, the lifting force cannot be applied directly along the center of gravity of the test piece throughout the operation. Instead, the only place where the lifting force can be applied is at the farthest horizontal edge of the test piece, which does not extend into the test tank during insertion. Because the lifting force is applied at a point which is displaced horizontally from the center of gravity, the application of this force creates a torque tending to rotate the test piece about its center of gravity, such rotation preventing the close-tolerance alignment which is required within the test tank. In order to compensate for this effect, the prior art has attached balancing weights which are suspended outward from the outermost edge of the test piece. These balancing weights shift the center of gravity of the test piece so that it corresponds to the edge along which the lifting force is applied.
However, the weight-attachment system has several drawbacks. First, the addition of balancing weights increases the total weight of the test piece, requiring higher capacity lifting devices. This problem can be alleviated to some degree by lengthening the arm where the balancing weight is applied; however, additional arm length brings about a second drawback. Often, the horizontal clearance to the side of the insertion point for the test tank is barely sufficient for the test piece. In which case, the arm of the balancing weight must be made extremely short. As the length of the balancing arm decreases, the amount of additional weight which must be applied to shift the center of gravity increases proportionately. For very short balancing arms, the balancing weights must be many times the weight of the original test piece. No prior art lifting devices exist which allow for precision lifting of test pieces without requiring the application of balancing weights as described.